Conventionally, to make an orifice it has been commonly known that an orifice hole is made directly on an orifice plate by means of mechanical processing and the like, wherein the orifice plate is interposed in a pipe passage at an appropriate position on the joint portion of the pipe passage or the connecting portion of a device and a pipe passage, and the orifice plate is directly clamped and fixed hermetically in place. However, with the afore-mentioned orifice of the directly clamped fixing type there is potential for deformation to be caused on the orifice plate due to clamping and fixing of the plate, thus making it difficult to make an orifice plate that is advantageously thin. For this reason, it is difficult to manufacture a high precision orifice having a prescribed hole diameter and shape, without uneven flow rate characteristics, and with stability and at low cost when a thin orifice plate is used.
To solve the afore-mentioned difficulties, an orifice has been developed wherein an orifice plate is welded to appropriate holding hardware, and the orifice plate welded to holding hardware is inserted into a pipe passage and fixed therein. However, with this solution there have arisen certain shortcomings such as that the orifice hole diameter may be altered by influence of heat caused at the time of welding, thermal stress might cause cracks on the thin orifice plate, and, furthermore, corrosion resistance of the orifice plate is low.
As seen above, an orifice, particularly a high precision orifice having certain constraints as to shape, hole diameter, and the like, which is employed with a pressure type flow rate control apparatus and the like cannot be manufactured at low cost, and at the same time it has been found that it is structurally difficult to insert and fix the orifice assembly in a pipe passage, thus causing various problems in practical use.
On the other hand, a pressure type flow rate control apparatus, which includes an orifice as a dispensable constituent member, has excellent characteristics in responsivity, control accuracy, manufacturing cost, maintainability and the like when compared with those same characteristics of a thermal type mass flow rate control apparatus (MFC) represented by a mass flow controller, and such as is widely used in the technical field of semiconductor manufacturing.
FIG. 7(a) and FIG. 7(b) show a basic block diagram of the afore-mentioned conventional pressure type flow rate control apparatus FCS The major portion of the pressure type flow rate control apparatus FCS comprises a control valve 2, a, pressure detector 6, an orifice 8, flow rate computation circuits 13, 31, a flow rate setting circuit 14, a computation control circuit 16, a flow rate output circuit 12, and the like. In FIG. 7(a) and FIG. 7(b), 3 designates an orifice upstream side pipe; 4 designates a valve actuator; 5 designates an orifice downstream side pipe; 7 designates a temperature detector; 9 designates a valve; 15 designates a flow rate conversion circuit; 10, 11, 22, 28 designate amplifiers; 17, 18, 29 designate ND converters; 19 designates a temperature correction circuit; 20, 30 designate computation circuits; 21 designates a comparison circuit; Qc designates a computation flow rate signal; Qf designates a switching computation flow rate signal; Qe designates a flow rate setting signal; Qo designates a flow rate output signal; Qy designates a flow rate control signal; Po designates gas supply pressure; P1 designates orifice upstream side gas pressure; P2 designates orifice downstream side gas pressure; and k designates a flow rate conversion rate.
One kind of pressure type flow rate control apparatus FCS, as shown in FIG. 7(a), is mainly used when the ratio P1/P2 of the orifice upstream side gas pressure P1 and the orifice downstream side gas pressure P2 is equal to the critical value of the fluid or lower than the critical value (that is, when the gas flow is under critical conditions). The gas flow rate Qc going through the orifice 8 is given by Qc=KP1 (where K is a proportional constant). Another kind of pressure type flow rate control apparatus FCS, as shown in FIG. 7(b), is mainly used for the flow rate control of gas during flow conditions both in critical and non-critical conditions. The gas flow rate. Qc going through orifice 8 is given, in this case, by Qc=KP2m(P1−P2)n, (where K is a proportional constant, and m and n are constants).
Furthermore, with the afore-mentioned pressure type flow rate control apparatuses, as shown in FIG. 7(a) and FIG. 7(b), the flow rate setting value for the flow rate control is given by a voltage value with a flow rate setting signal Qe. For example, when the pressure control range is 0˜3 (kgf/cm2 abs) for the upstream side pressure P1 is provided by the voltage range 0˜5 V, Qe=5V (a full scale value) becomes equivalent to the flow rate Qc=KP1 in the pressure P1 of 3 (kgf/cm2 abs).
Specifically, if a flow rate setting signal Qe=5V is inputted when the conversion rate k of the flow rate conversion circuit 15 is set at 1, a switching computation flow rate signal Qf (Qf=kQc) becomes 5V, and a control valve 2 is operated to open or close until the upstream side pressure P1 reaches 3 (kgf/cm2 abs), thus resulting in that gas of a flow rate Qc=KP1, corresponding to P1=3(kgf/cm2 abs), flows through the orifice 8.
In the case that the range of pressure to control is switched to 0˜2 (kgf/cm2 abs), and the pressure range is provided by the flow rate setting signal Qe of 0˜5V (that is, when a full scale value of 5V gives 2(kgf/cm2 abs)), the afore-mentioned flow rate conversion rate k is thus set at ⅔.
As a result, assuming that the flow rate setting signal Qe=5V is inputted, because of the requirement that Qf=kQc, the switching computation flow rate signal Qf becomes Qf=5×2/3V, and the control valve 2 is operated to open or close until the upstream side pressure P1 becomes 3×2/3=2 (kgf/cm2 abs). In other words, the full scale flow rate is converted so that Qe=5V shows the flow rate Qc=KP1 equivalent to P1=2 (kgf/cm2 abs). It is also the same with the afore-mentioned pressure type flow rate control apparatus shown in, FIG. 7(b). In this case, the flow rate Qc of gas passing through orifice 8 is given by Qc=KP2m(P1−P2)n, (where K is a proportional constant, and m and n are constants). The afore-mentioned proportional constant K changes when the type of gases is changed.
As stated above, the conventional pressure type flow rate control apparatus is constituted so that the flow rate Qc=KP1, under the orifice upstream side pressure P1 corresponding to Qe=5V (full scale•F.S.), can be switched by adjusting the conversion rate k of the flow rate conversion circuits 15. However, the flow rate switching range is strictly limited to within the range of the flow rate smaller than the flow rate Qc=KP1 under the upstream side pressure P1 corresponding to the afore-mentioned Qe=5V (F.S. value), and the flow rate Qc under the upstream side pressure P1 becomes the flow rate value unequivocally determined by a constant k obtained given a particular orifice diameter, and the like.
In other words, in order for the pressure control range 0˜3 (kgf/cm2 abs) under the orifice upstream side pressure P1 to be provided by the voltage range 0˜5V, and the flow rate value Qc at Qe=5V is switched up to the flow rate, for example, 5 times, it becomes necessary that the orifice itself needs to be changed to a new orifice corresponding to a constant K having a value 5 times that of the previous orifice.
FIG. 8 shows an example of the afore-mentioned pressure type flow rate control apparatus, which has structure making it possible to replace the orifice, wherein an orifice insertion hole 24 is provided on the fluid outlet side of a valve body 23 of the control valve 2, which forms a pressure type flow rate control apparatus FCS. With this structure, an orifice 8 having an appropriate diameter is inserted into the orifice insertion hole 24, and hermeticity of the fluid passage 34 is secured through the mediation of pressing metal ware 32, a bearing 33 and a sealing material 35a. 
Patent Document 1: TOKU-KAI-HEI No. 8-338546
Patent Document 2: TOKU-KAI No. 2000-66732
Patent Document 3: TOKU-KAI No. 2000-322130
Patent Document 4: TOKU-KAI No. 2003-195948
Patent Document 5: TOKU-KAI No. 2004-199109
Now, with the afore-mentioned conventional pressure type flow rate control apparatuses, as shown in FIG. 7(a) and FIG. 7(b), because supply pressure Po for the fluid to be controlled is ordinary maintained at a set value, the maximum value Qc of the flow rate to be control is unequivocally determined by the orifice 8 that will be used. So, unless the orifice itself is replaced, it is difficult to change the maximum or minimum flow rate Qc of the afore-mentioned fluid in a large scale.
Also, according to the apparatus of FIG. 8, it is necessary that the pressing metal ware 32 be removed each time the orifice 8 is replaced, and in some cases when the sealing material 35a is changed. However, to dismantle the piping passage is time consuming and difficult.
Furthermore, when using the orifice replacement method in accordance with the apparatus of FIG. 8, it is very difficult to completely prevent an outside leak from the so-called orifice sealing part because the apparatus is constituted so that the sealing mechanism of orifice 8 comprises the sealing material 35a attached to the surface side of the orifice 8 and the touching seat 35b for sealing on the valve main body side attached to the reverse face side of the orifice 8. Furthermore, the sealing characteristics of both faces of the orifice 8 are secured by the screw-in pressing pressure of the pressing metal ware 32.
In addition, another shortcoming is that the orifice plate cannot be very thin because, as stated before, the orifice 8 is directly clamped and fixed in place as shown in FIG. 8. Thus, to machine-process a very thin metal made orifice plate to make the orifice with a prescribed inside diameter thereon is difficult to manufacture with both a high-precision orifice and at low cost.
Another difficulty is, as stated before, that distortion, cracks, corrosion, and the like, caused on the orifice plate by weld heat cannot be prevented even though the orifice plate is constituted so that orifice holding metal ware is welded to a thin orifice plate, and the orifice holding metal ware is positioned and fixed.